Developments in higher wattage, compact integral lighting systems (i.e., integral lamp and ballast systems) are resulting in ballast heat densities that test the limits of traditional free-convection cooling methods. Disadvantageously, discrete component life is limited by operating temperature. In an exemplary application, an integral ceramic metal halide (CMH) lamp uses a heat sink to transfer heat from the devices comprising the electronic ballast to the inside wall of the ballast""s plastic housing. Typically, there are significant air gaps between the fins and interior walls, which gaps reduce the effectiveness of heat removal as temperature of the electronic devices increases. Furthermore, the increase in temperature ultimately reduces the overall reliability of the ballast electronics.
Accordingly, there is a need for innovative thermal management and packaging techniques capable of handling higher heat dissipation requirements and for enabling higher-power and more compact integral lamps.
A heat sink for a compact, integral lamp system comprises a bimetal strip or a bimetal heat pipe. The bimetal strip comprises two thin strips of metal bonded lengthwise together. With temperature change, the bimetal strip flexes toward one metal or the other, depending on the physical properties of the particular metals. The deformation of the strip causes heat sink fins to press against the surface of the ballast wall, thereby significantly reducing or eliminating the air gaps between the heat sink fins and ballast wall and thus more effectively removing heat from the ballast.
A bimetal heat pipe comprises two pieces of metal bonded together to form a pipe with a liquid contained therein. With temperature change, the bimetal pipe, or container, flexes in a certain direction, depending on the physical properties of the particular metals. The metal that expands more under heat will bend toward the other metal, causing a physical deformation of the pipe. The liquid inside the bimetal pipe tends to boil at the hot end and condense at the cool end, thereby transferring heat away from the ballast components.